Despite an exponential increase over decades in research and development (R&D) expenditures by the pharmaceutical industry, the average number of new drugs approved annually by the FDA, 25 or so, has remained stagnant. The unsustainable trend of runaway spending in R&D directly contributes to skyrocketing health care costs across the country, underscoring an urgent need for new drug targets and classes. Protein-protein interactions (PPIs) control all aspects of cellular processes and are considered to be the ?Holy Grail? in drug discovery and ultimate drug targets for disease intervention. Unfortunately, neither of the two major classes of existing drugs ? low molecular weight compounds and antibodies are effective in interfering with intracellular PPIs. On the other hand, although small peptides are capable of effectively disrupting PPIs, they generally lack drug-like properties due primarily to their susceptibility to proteolytic degradation in vivo. A sensible solution to this long-running enigma in drug discovery is side chain-stapled L-peptides and proteolysis-resistant D-peptides. Antagonizing MDM2/MDMX to activate p53 is one of the most promising therapeutic paradigms for anticancer therapy. We have recently developed a series of high-affinity L- and D-peptide antagonists of MDM2 that effectively suppress tumor growth in vitro and in vivo in a p53- and MDM2-dependent manner. To achieve sustained and robust p53 activation and optimal therapeutic efficacy, however, dual-specificity antagonists of both MDM2 and MDMX are needed. The overall goal of this proposed research is to develop ultrahigh- affinity, dual-specificity, and proteolysis-resistant peptide antagonists of MDM2 and MDMX as a powerful p53- activating modality for the treatment of acute myeloid leukemia ? where mutations of the TP53 gene are rare and MDM2 and MDMX are often amplified or over-expressed. This project may lead to the addition of new weapons to the existing anticancer arsenal and broadly impact the development of peptide therapeutics for targeted molecular therapy of many other diseases as well.